Solid state detectors have been developed for measuring anhydrides, i.e., SO.sub.X, NO.sub.X, etc., present in monitored gas environments such as the stack, or flue, gas environments of a combustion process. The solid state detectors include a solid electrolyte element containing oxy-anions of the particular anhydride to be detected. In most practical industrial applications, the detector requires compensation for changes in the oxygen content of the monitored gas environment and indeed it is often necessary to separately measure the oxygen content of the monitored gas environment. A suitable solid state sensor for anhydrides is described in U.S. Pat. No. 4,282,078, which has been assigned to the assignee of the present invention, and is incorporated herein by reference. The use of oxygen ion conductive solid electrolyte sensors for oxygen measurements is well known and is described in detail in the U.S. Pat. No. Re. 28,792, which is assigned to the assignee of the present invention and incorporated herein by reference.